Cathode including a non fluorinated linear chain polymer as the binder, method of making the cathode, and lithium electrochemical cell containing the cathode

ABSTRACT

A cathode suitable for use in a lithium electrochemical cell is made from aixture of active cathode material, carbon, and non fluorinated linear chain polymer by a method including the steps of 
     (A) dissolving the non fluorinated linear chain polymer in a non polar solvent at a temperature near the melting point of the polymer, 
     (B) adding the active cathode material and carbon and evaporating the solvent, and 
     (C) grinding the dried mixture into a fine powder and making it into a cathode by pressing the powdered mixture onto both sides of an expanded metal screen and then cutting to the desired dimensions. 
     The cathode can be combined with lithium as the anode and a solution of 0.8 mol dm -3  LiAlCl 4  in a mixed organic solvent of 24 mass percent 4-butyrolactone in 1, 2 dimethoxyethane as the electrolyte to provide a mechanically stable, relatively inexpensive lithium electrochemical cell having good cell performance.

The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to us of any royalty thereon.

This application is a division of application Ser. No. 749,597, filed June 27, 1985.

This invention relates to a cathode including a non fluorinated linear chain polymer as the binder, to a method of making the cathode, and to a lithium electrochemical cell containing the cathode.

BACKGROUND OF THE INVENTION

Existing technology for the fabrication of cathodes for use in lithium primary and secondary cells utilizes Teflon as the binding material. Teflon or polytetrafluoroethylene is expensive although inert, and its use results in cathode structures of poor mechanical stability. These problems do not easily lend themselves to the large scale production of cathodes in manufacturing.

SUMMARY OF THE INVENTION

The general object of the invention is to provide a cathode suitable for use in lithium electrochemical cells. Another object of the invention is to provide such a cathode that will be mechanically stable and relatively inexpensive to manufacture. A further object of the invention is to provide such a cathode that will be suitable for use in a primary or secondary lithium cell.

It has now been found that the aforementioned objects can be attained by replacing the Teflon binder with a ncn fluorinated linear chain polymer such as polypropylene (PP) or polyethylene (PE).

More particularly, according to the invention, a cathode for use in lithium primary or secondary cells can be prepared from a mixture of active cathode material, conductive dilutant such as carbon and non fluorinated linear chain polymer. The amount of carbon can be varied depending upon the resistivity of the cathode material and desired porosity of the final cathode structure. Typical cathode materials that can be used are metal halides, oxides and sulfides. The polymer is first dissolved in a non polar solvent such as Decalin (Decahydronaphthalene) or tetrachloroethylene at a temperature near the melting point of the polymer (100° to 150° C.). The active cathode material and carbon are added and the solvent evaporated.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following is a preparation of a cathode structure utilizing TiS₂ as the active cathode material. The procedure is performed in an argon filled dry box. Polypropylene powder is dissolved near its crystalline melting temperature of about 100° to 120° C. in a small volume of decahydronaphthalene of about 5 mils decahydronaphthalene for about 0.1 to 0.2 gm (PP), the solution being stirred continuously during heating. Once the (PP) is dissolved, the solution is removed from the heat and cooled below 100° C. and the active cathode material and carbon powders are then added quickly before the polymer solution cools completely. The powdered materials and solutions are stirred vigorously by hand until the solution is absorbed and the mix becomes granular and has cooled to room temperature. The mixture is then dried in a vacuum oven at about 120° to 150° C. for 12 hours in order to remove the decahydronaphthalene. The dried mixture is then ground into a fine powder and made into cathodes by pressing the powdered mixture onto both sides of an expanded metal screen and then cut to desired dimensions.

Although flat plate type electrodes have been prepared in the foregoing embodiment to demonstrate the use of the non fluorinated linear chain polymers as binding materials, the method results in moderately flexible structures which makes the method equally adaptable to the preparation of rolled electrodes, either cold-rolled or rolled through heated rollers using a powdered mix or a slurry mixture using a non reactive organic solvent.

The TABLE shows typical results for a Li-TiS₂ cell in which cell performance is studied as a function of solvent composition, cathode composition, and cathode preparation, that is, cold-pressing or hot-pressing. Performance is found to be equal to that obtained from cells utilizing cathodes of poor mechanical properties, that is, based on teflon binders. The TABLE shows that one can operate with a wide range of (PP) content in the electrode. More specifically, one can use less binder and still obtain a good mechanically stable cathode. In fact, one can go down to as little as 1 to 3 weight percent of binder which allows one to put in more active cathode material and improve the performance of the cell.

                                      TABLE                                        __________________________________________________________________________     Performance Data for Li--TiS.sub.2 Cells Utilizing                             Polypropylene (PP) as the Cathode Binder.sup.a.                                Electrode           Mass %                                                                              Mass %                                                                              Mass %                                                                              Current  Cathode                            Type.sup.b                                                                            t/oc                                                                               Electrolyte                                                                             TiS.sub.2                                                                           Carbon                                                                              PP   Density: mA/cm.sup.2                                                                    Efficiency                         __________________________________________________________________________     cold-pressed                                                                          25  0.8 mol dm.sup.-3                                                                       80   10   10   5.0      49.3                                      25  LiAlCl.sub.4            2.0      75.2                                      -20 in                      2 0      39.5                                      -30 24% 4-BL/DME            2.0       5.6                               cold-pressed                                                                          25  1.6 mol dm.sup.-3                                                                       78.5 10   11.5 2.0      66.7                                      25  LiAsF.sub.6 in          1.0      82.0                                          2Me--THF                                                            cold-pressed                                                                          25  1.3 mol dm.sup.-3                                                                       83.3 9.6  7.1  1.0      71.1                                          LiAsF.sub.6 in                                                                 2Me--THF                                                            cold-pressed                                                                          25  1.6 mol dm.sup.-3                                                                       81.8 12.7 5.5  2.0      69.5                                          LiAsF.sub.6 in                                                                 2Me--THF                                                            cold-pressed                                                                          25  1.2 mol dm.sup.-3                                                                       87.3 10.0 2.7  1.0      80.2                                          LiAsF.sub.6 in                                                                 2Me--THF                                                            cold-pressed                                                                          25  0.85 mol dm.sup.-3                                                                      79.7 10.1 10.2 2.0      68.9                                          LiAlCl.sub.4 in                                                                24% 4-BL/DME                                                        __________________________________________________________________________      .sup.a 2Me--THF is 2methyl tetrahydrofuran                                     DME is 1,2dimethoxyethane                                                      4BL is 4butyrolactone                                                          .sup.b Cathode porosity in the range of 40-60%                           

DESCRIPTION OF THE DRAWING

The drawing illustrates a cycling profile for a rechargeable lithium cell using a TiS₂ cathode and an electrolyte consisting of 0.8 mol dm⁻³ LiAlCl₄ in a mixed organic solvent of 24 mass percent 4-butyrolactone in 1,2 dimethoxyethane (24% 4-BL in DME). The cathode (80 mass % TiS₂, 10 mass % carbon and 10 mass % (PP) is prepared as described in the description of the preferred embodiment.

Referring to the drawing, the cell used for the drawing is cycled at 25° C. at a current density of 2.0 mA cm⁻², and excursions are shown for discharges at 5.0 mAcm⁻² and at 2.0 mAcm⁻² at lower temperatures of -20° C. and -30° C. Cycling is stopped after 33 cycles. The drawing also shows that after excursions to higher current densities and/or lower temperatures, cell performance recovers exceptionally well. In the drawing, the ordinate, percent cathode utilization, is an indicator as to how the cell is performing.

Any non fluorinated linear chain polymer can be used in the cathode that is stable in the electrolyte of the lithium cell. The linear chain polymers are inert in a wide variety of non aqueous solvents including ethers and lactones. Suitable polymers include (PP) and (PE).

Active cathode materials that can be used include metal halides, metal oxides, and metal sulfides of which TiS₂ is preferred. Pure carbon can be used for those cells in which the solvent serves as the depolarizer.

Any carbon black can be used as the carbon for the cathode that enhances the conductivity of the electrodes. The particular carbon black used in the description of the preferred embodiment is Shawinigan Black but other high surface area carbons or graphite can also be used. The invention even contemplates a cathode made from a mixture of polymer and active cathode material with no carbon present.

The electrolyte used in the lithium cell must be compatible with the cathodes made according to the invention. Suitable electrolytes include a solution of an inorganic lithium salt in a pure or mixed organic solvent.

In the method of the invention, mechanically stable structures can be easily prepared by cold-pressing or cold rolling, and by hot-pressing or hot-rolling. Moreover, sintering temperatures below 170° C. should be used since some cathode materials are subject to decomposition above 200° C.

Thus, it has been demonstrated that non fluorinated linear chain polymers such as (PP) or (PE) can be used to prepare mechanically stable cathodes for nonaqueous lithium cells. The use of the non fluorinated linear chain polymers as binders results in low cost cathodes giving equal electrochemical performance as do Teflon bonded cathodes, but the use of the non fluorinated linear chain polymers results in cathodes having great mechanical stability that can be fabricated in several forms such as plates or rolls, and that can be made as thin as 0.5 mm or less.

We wish it to be understood that we do not desire to be limited to the exact details as described for obvious modifications will occur to a person skilled in the art. 

What is claimed is:
 1. Method of preparing a cathode for use in a lithium electrochemical cell from a mixture of TiS₂, carbon and polypropylene, said method including the steps of(A) dissolving polypropylene at 100° to 130° C. in a small volume of decahydronaphthalene contained in an argon filled dry box while stirring the solution continuously during heating, (B) removing the solution from the heat, cooling to below 100° C., and quickly adding the carbon and TiS₂ before the polymer solution cools completely, (C) stirring the powdered materials and solution vigorously until the solution is absorbed and the mix becomes granular and has cooled to room temperature, (D) drying the mixture in a vacuum oven at 120° C. to 150° C. for 12 hours in order to remove the decahydronaphthalene, and (E) grinding the dried mixture into a fine powder and making into a cathode by pressing the powdered mixture onto both sides of an expanded metal screen. 